The invention is generally related to floating offshore structures and more particularly to the load out, float off, and channel tow of a spar type hull.
There are a number of spar hull designs available in the offshore drilling and production industry. These include the truss spar, classic spar, and cell spar. The term spar hull structure described herein refers to any floating structure platform, which those of ordinary skill in the offshore industry will understand as any floating production and/or drilling platform or vessel having an open centerwell configuration.
The spar supports a topside structure and comprises a hard tank, truss section, and a soft tank. In the case of the classic spar, the hard tank and soft tank are connected by a cylinder instead of a truss. The hard tank supplies the majority of the buoyancy to support the hull structure, risers, and topsides. The hard tank is divided into a number of chambers among which the ballast water can be shifted to control the spar's buoyancy and stability.
When the spar is placed in its operating configuration offshore, the spar cylinder is exposed to currents in the ocean. The current acting on the spar cylinder produces VIV (vortex induced vibration). Because the VIV can produce unacceptable motions of the spar, helical strakes are added to the cylindrical portion of the spar as a means of eliminating or reducing the VIV. The strakes extend outwardly from the hard tank and are attached in a helical pattern around the hard tank. The fact that helical strakes reduce VIV is well known in the offshore industry.
The hard tanks of a spar can be as much as 150 feet in diameter. To be effective the strakes must extend outward from the hull a distance of 12-15% of the hull diameter. The strakes add significantly to the outside diameter of the hard tank without adding much buoyancy. Spars are built lying on their sides, loaded out onto HLV's (Heavy Lift Vessels) on their sides, and floated off into the water on their sides. Therefore, on the larger diameter spars there is not sufficient water depth near the fabrication yard to provide bottom clearance for the strake tips. Since the float off operation is very sensitive to sea states, the spars must be floated off the HLV in protected water near the fabrication yard.
When the HLV is ballasted downward to float off the spar, the HLV with the spar on board goes through a minimal stability when the deck of the HLV goes awash. This occurs because the HLV loses most of its water plane area when its deck goes awash, and the spar is not yet picking up much water plane area. Traditionally, the problem has been solved with two methods. First, the HLV is trimmed by the stern so that the soft tank of the spar picks up some water plane area, before the HLV deck goes awash. Second, supplementary stability modules are added to the HLV deck to improve stability.
Frequently, after the spar is floated off the HLV in protected water the spar must be moved alongside the fabrication yard quay to perform additional work. Once the spar is completed it must be towed down the channel to the open sea. However, there is insufficient water depth at quayside to provide strake tip clearance, and there is insufficient strake tip bottom clearance in the channel leading from the fabrication yard to the open sea.
One method of solving the clearance problem is to install a portion of the strakes at sea. That way, the strakes do not project below the “belly” of the hard tank during the movement of the spar to quayside, or during the channel tow. This method has been tried once, and was found to be more difficult and expensive than expected.
The weight and vertical center of gravity of a large diameter spar are too great for load out and float off by existing HLV's. The solution has been to build the spar in two pieces, then load out and transport each piece separately, float off the individual pieces, and then join them as they float on their sides in protected water. However, this adds expense and difficulty to the construction of the spar and is not a favored solution.
Thus, it can be seen that there is a need for an improved method for load out and transport to the open sea of a spar type structure.